In an ordinary ring box was a not so ordinary ring. This ring was cursed by a wizard many centuries ago. It gave the wearer the ability to transfer anything about one person. From their ages (one would get older while the other younger), height (one shrank while the other grew), weight (thinner and fatter), iq (smarter and dumber), anything that was plausible. All the person had to do was will it with both targets in mind. Due to how complex the changes could be. History would be rewritten to fit the change. Only person who'd know how things should be was the wearer of the ring and if they please the other person getting a trait transfered.
The ring was thought to have been lost through time. Being discarded, ignored, handed down to generation to generation. Till modern times when the ring's power was discovered by a...